Celestial sightings provide information that can be used to derive platform attitude, direction, and position. Celestial navigation techniques have been in use since ancient times, with stars originally providing direction information to voyagers, e.g., Polaris. Until the 1950s, the elevation angles would be measured against a vertical reference provided by the horizon at sea. Subsequent mechanizations of the process rely on inertial instruments to maintain knowledge of the local vertical when the horizon is obscured or when the measurement is done from a moving platform such as an aircraft. However, inherent drift of inertial sensors limits accuracy of the vertical measurement, and ultimately system performance.
It has long been known that it is possible to derive one's position by observing an orbiting satellite against a star-field background. Angles between an apparent position of the satellite and (assumed known) stars contain information on the location of the observer. This method requires the position of the satellite be known accurately. Given that satellite orbits can be predicted using well-known physical equations, e.g., Kepler's and Newton's laws, this technique can provide a practical method of navigation.
A technique, dubbed “SkyMark,” was developed at The Charles Stark Draper Laboratory, Inc. (“Draper”) four decades ago and has been closely investigated since then for use in a variety of applications, including autonomous spacecraft navigation. FIG. 1 schematically illustrates principles of operation of SkyMark. Draper has been a pioneer in SkyMark with published works on SkyMark that include a Draper Fellow Master's MIT thesis (Willhite, W. B., An Analysis of ICBM Navigation Using Optical Observations of Existing Space Objects, CSDL-T-1484, Masters Thesis, Massachusetts Institute of Technology, June 2004, the entire contents of which are hereby incorporated by reference), and in Shearer, J., et al., “SkyMark—A Technique for Enhancing Precision Missile Navigation,” AIAA Missile Sciences Conf., (Nov., 2004), which is also incorporated herein by reference.
As depicted in FIG. 1, a star tracker (not shown in FIG. 1) associated with a vehicle 100 is used to measure a direction (indicated by ray 102) toward a celestial object 104 (in the case shown, a fixed star) and a direction R1 toward a skymark 106 at one or more instants of time, noting that the terms “star” and “skymark” are defined below. The angle between the two directions 102 and R1 is designated θ. A star tracker may use information in a star catalog to locate itself, and its associated vehicle, in space.
The concept of using satellite sightings together with inertial guidance for navigation purposes was discussed by Brown et al. in “Long Duration Strapdown Stellar-Inertial Navigation using Satellite Tracking,” Position Location and Navigation Symposium, 1992. Record. 500 Years after Columbus-Navigation Challenges of Tomorrow. IEEE PLANS'92, IEEE, pp. 194-201 (1992), which is incorporated herein by reference.
While the concept of using satellite sightings for ownship positioning is known, improved methods and apparatus for collecting data, processing the data and integrating such methods and apparatus into complete navigational solutions are needed to overcome limitations imposed by the paucity of trackable objects subject to operational conditions. Thus, using methods that are currently known, requisite accuracy cannot be achieved under certain conditions.